The present invention is in the field of electrical power systems and, more particularly, power conversion and distribution systems which operate in vehicles such as aircraft for purposes of performing electrical main engine starts.
In a typical aircraft, electrical power may be distributed with a 28 volt direct current (Vdc) system. In prior-art aircraft, main engine starting may be performed electrically or pneumatically. In the case of prior-art electrical starting, a DC starter-generator may be connected to a 24 Vdc battery. In the case of prior-art pneumatic starting, an air-turbine starter motor may be driven from a high pressure air source. As aircraft designs improve, these prior-art starting techniques require improvement.
One major field of design improvement is known in the aircraft industry as More Electric Aircraft (MEA). In the context of MEA design, pneumatic starting systems are considered undesirable. Consequently, electrical starting systems have become the systems of choice in MEA designs. It is also anticipated that in the near future brushless starter-generators will replace the brush type starter-generators which have been routinely used on aircraft with 28 Vdc distribution systems.
In addition to a MEA design evolution, there is a design evolution in a direction of more efficient, higher power engines. These newer engines require increased torque and speed for starting. Prior-art electrical starting systems may not provide the requisite torque and speed to efficiently start newer engines.
Another consideration is that main engines started pneumatically depend upon the availability of the aircraft auxiliary power unit (APU) as a source of high pressure air. When the APU is inoperative, a backup source of high pressure air must be located, usually in the form of a ground cart. These pneumatic ground carts are not as readily available as electric ground carts, and hence aircraft with electric start systems are considered easier to dispatch.
Various efforts have been made in the prior-art to improve electrical starting systems. In one prior-art example, two 24 Vdc batteries are connected in series to drive an electrical starter with 48 Vdc. This arrangement requires use of multiple contactors. Also electrical isolation is needed to assure that 48 Vdc is not applied to any equipment on the aircraft which may not be tolerant of 48 Vdc. This isolation is typically provided through use of a dedicated starter bus which is electrically separable from main power distribution buses of the aircraft.
Such a prior art system may be understood by referring to FIG. 1. In FIG. 1, a prior art power distribution system is designated generally by the numeral 10. The system 10 may be a starter and system for engines 12 and 14 of an aircraft (not shown). The system 10 may also be a generator system for the aircraft. The system 10 may comprise starter-generators 16 and 18, control units 20 and 22, aircraft power buses 24 and 26 and power sources or batteries 28 and 30.
The prior-art system 10 may be configured so that, in a power generation mode, direct current (DC) power may be provided to the aircraft power buses 24 and 26 at a voltage that is approximately equal to an output voltage of the batteries 28 and 30 and the starter-generators 16 and 18. In a typical aircraft, the voltage on the buses 24 and 26 may be about 28 Vdc. Various electrical devices may be connected to the buses 24 and 26. These devices are symbolically represented as blocks numbered 32 and 34. In a typical aircraft the devices 32 and 34 may be rated to operate at a 28 Vdc bus voltage.
The prior-art system 10 may also be configured to provide engine starting at a voltage higher than the 28 Vdc bus voltages. This may be accomplished by connecting both of the batteries 28 and 30 in series and employing the series connected batteries 28 and 30 to start the engines sequentially. For example the engine 12 may be started by driving the starter-generator 16 using a 48 Vdc power source. In this starter mode of operation, provision must be made to avoid energizing the aircraft power buses 24 and 26 with the 48 Vdc starting voltage.
In order to avoid applying 48 Vdc to the aircraft power buses 24 and 26, a start bus 36 may be provided in the system 10. It may be seen that various contactors and connection paths may be employed to perform engine starting through the start bus 36. The following Table 1 explains how various combinations of contactors shown in FIG. 1 and their respective switching states may provide requisite current routing of the prior-art system 10.
TABLE 1(Prior Art)StartStartPowerContactorEngineEngineGenerationNumber1214ModeLC1OpenclosedLC2openclosedSC1closedopenopenSC2OpenclosedopenSC3closedclosedopenBC1OpenopenclosedBC2closedBC3Openopenclosed
In another prior-art example an auxiliary power unit (APU) generator may be connected in series with a battery to produce a 48 Vdc starting voltage. In this arrangement the APU generator must be designed to produce the high currents needed for starting. Such a robust APU generator may not be desirable on an aircraft because of its size, weight and cost. Multiple contactors and buses are also required to temporarily interconnect the APU generator and the battery and preclude application of 48 Vdc to 28 Vdc rated equipment.
As can be seen, there is a need to provide a system of electrical power distribution and control that enhances operation of an electrical engine starter. Additionally, there is a need to reduce the number, size and weight of components used to control electrical power distribution for such enhanced operation of the engine starter.